linux-kernel - Re: [RFC] de-asmify the x86-64 system call slowpath

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Message-ID: <CA+55aFyajjkCtRiTOS2AbFOWi7qt2FUnW6T3iG9nXqoZ0O9Ljg@mail.gmail.com>
Date:	Mon, 27 Jan 2014 14:43:14 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Andy Lutomirski <luto@...capital.net>, Peter Anvin <hpa@...or.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] de-asmify the x86-64 system call slowpath

On Mon, Jan 27, 2014 at 2:32 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> do_signal() is also a place where arbitrary changes to regs might've
> been done by tracer, so regs->cs might need to be checked in the same
> place where we validate regs->rip ;-/

Fair enough. But it would still be really easy, and make the common
case signal delivery a bit faster.

Now, sadly, most signal delivery is then followed by sigreturn (the
exceptions being dying or doing a longjmp), so we'd still get the
iretq then. But it would cut the iretq's related to signals in half.

We *could* try to do sigreturn with sysret and a small trampoline too,
of course. But I'm not sure how far I'd want to take it.

               Linus
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